The present invention is directed to aqueous ink-jet compositions for printing on commercial offset coated media.
In recent years, computer printer technology has evolved to a point where very high resolution images can be transferred to various media. One particular type of printing involves the placement of small drops of a fluid ink onto a surface in response to a digital signal. Typically, the fluid ink is placed or jetted onto the surface without physical contact between the printing device and the surface. The method that the ink is deposited onto the printing surface varies from system to system. However, two major systems include continuous ink deposit and drop-on-demand ink deposit.
With regard to continuous printing systems, inks used are typically based on solvents such as methyl ethyl ketone and ethanol. Essentially, continuous printing systems function as a stream of ink droplets are ejected and directed by printer nozzle. The ink droplets are directed additionally with the assistance of an electrostatic charging device in close proximity to the nozzle. If the ink is not used on the desired printing surface, the ink is recycled for later use. Regarding drop-on-demand printing systems, the ink-jet inks are typically based upon water and glycols. Essentially, with these systems, ink droplets are propelled from a nozzle by heat or by a pressure wave. Additionally, all of the ink droplets are used to form the printed image and are ejected when needed.
There are several reasons that ink-jet printing has become a popular way of recording images on various media, particularly paper. Some of these reasons include low noise, capability of high speed recording, and multi-color recording. Additionally, these advantages can be obtained at a relatively low price to consumers. However, though there has been great improvement in ink-jet printing, accompanying this improvement are increased demands on ink-jet printing, e.g., higher speed, higher resolution, full color image formation, etc.
There are several characteristics to consider when evaluating a printer ink in conjunction with a printing surface or substrate. Such characteristics include (a) edge acuity and optical density of the image on the surface, (b) dry time of the ink on the substrate, (c) adhesion to the substrate, (d) lack of deviation of ink droplets, (e) presence of all dots, (f) resistance of the ink after drying to water and other solvents, (g) long-term storage stability, and (h) long-term reliability without corrosion or nozzle clogging, to name a few. Though the above list of characteristics provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above characteristics. Often, the inclusion of an ink component meant to satisfy one of the above characteristics can prevent another characteristic from being met. Thus, most commercial inks for use in ink-jet printers represent a compromise in an attempt to achieve at least an adequate response in meeting all of the above listed requirements.
Papers used for ink-jet printing have typically included high-quality or wood-free papers designed to have a high ink absorptivity. These papers are functionally good for ink-jet printing in some respects because the ink-jet inks may be absorbed readily and dry quickly. However, such papers often do not allow for a crisp or sharp image. In order to attain enhanced print quality and image quality such as is present in a photograph, specialty media has been developed to work with aqueous inks. For example, various coating techniques are currently being used which employ sol-gel and multi-layer coating systems, producing papers that are designed for use with aqueous ink-jet inks.
Conversely, with commercial offset paper, a nonporous smooth surface can provide a good printing surface for a crisp image. However, commercial offset coated papers are significantly different than office plain papers and photo or glossy papers specifically designed for ink-jet media. Typically, with commercial offset media, the smooth non-porous surface is formed by a coating which requires more time for fluids to penetrate. Because of this, more drying time is required. This is due in part to the diffusion-type adsorption typically required to occur for aqueous inks to dry (as opposed to the capillary-type absorption available for ink penetration of office papers and some ink-jet papers). In many instances, offset coatings contain polymers that are more hydrophobic, e.g., styrene-butadiene based, than paper coatings specifically designed for ink-jet ink, e.g., water-soluble polymers such as polyvinyl alcohol. Thus, because offset coatings are typically hydrophobic, have poor penetration properties, and are smooth/non-porous, offset coatings tend to interact poorly with water-based inks. Further, classic ink-jet solvents such as glycols and diols tend to perform poorly on these coatings, showing long drytimes and poor spreading characteristics. Examples of polymers used in offset media include latex binders, polystyrenes, polyolefins (polypropylene, polyethylene, polybutadiene), polyesters (PET), polyacrylates, polymethacrylates, and/or poly (maleic anhydride).
When using standard aqueous based ink-jet inks, as stated, poor performance on commercial offset media is typically encountered. For example, some dye based color inks penetrate deeply into offset paper and provide poor print quality as well as dull colors (low chroma). Other water-based inks tend to xe2x80x9cbloomxe2x80x9d or migrate horizontally over long periods of time. Additionally, many pigmented inks tend to dry very slowly, some taking more than 24 hours. In some instances, after 24 hours, the dried print samples remain tacky. Numerous other printing defects can also be present including coalescence, poor black-to-color, color-to-color bleed control, and halo. As a result, many known water-based ink-jet inks are unacceptable when used in conjunction with offset media such as commercial offset paper.
An aqueous ink-jet ink composition for printing on offset media is disclosed comprising an ink colorant, an effective amount of at least one surfactant, and an effective amount of at least one aprotic polar solvent wherein the solvent is comprised of molecules having a molecular weight from 40 to 1500. Optionally, the ink-jet ink compositions described herein can further comprise an effective amount of a non-aprotic polar solvent. The aprotic polar solvent(s) and the optional non-aprotic polar solvents used can independently have a polarity from 5 to 20 Debye (D).